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Ask Extension expert: Sorry. There’s no ‘one and done’ method to get rid of arum
Ask Extension expert: Sorry. There’s no ‘one and done’ method to get rid of arum
Ask Extension expert: Sorry. There’s no ‘one and done’ method to get rid of arum

Published on: 03/27/2023


Gardening season is just around the corner, and you may have questions as you prepare for the season. To ask one, simply go to the OSU Extension website, type it in and include the county where you live. A photo is very helpful.

Q:  Italian arum weed in my yard is worse than last year, even with extensive weeding. I’m thinking to try using vinegar. Should I spray it on the leaves or water it into the soil? There are other plants in some of the locations; how close/deep/much to use in the root method without hurting other plants would be helpful to know. – Washington County

A: Unfortunately, Italian arum is very hard to get rid of. No research supports using vinegar as an herbicide on this plant. Even highly corrosive horticultural vinegar wouldn’t affect the bulbs, and the leaves it burned off would soon grow back. The waxy leaves and many, rapidly dividing bulbs, plus caustic sap, make this plant very hard to manage. If you have a small patch, digging out the bulbs and surrounding soil is an option. If you have a larger patch, research shows that some herbicides can control this plant. Learn more here.

In my past experience as a natural areas steward, there is no “one and done” method of control for Italian arum. Ongoing management is needed and even then, you will likely never be completely rid of it.

Heavy mulching with impervious material could be a management strategy at a garden scale if you are able to keep it in place for years. I don’t find any research on this but have seen it used on a limited basis in ecological restoration.  I don’t know how long it takes the bulbs to die in the soil if they are deprived of light, or if they would send out shoots to try poke through the mulch or get out from under it. Some materials like old plywood and carpeting might introduce synthetic compounds into the soil and water. These compounds could potentially end up in food crops.

If growing organically is a priority you might consider checking out this fact sheet about what mulches are allowed for farmers growing certified organic produce. – Elza Records, OSU Extension education program assistant

Not all mulches are suitable for garden beds

Q: We received a number of bags of premium wood much and I am wondering if we can use it in our raised garden beds. I’ve tried searching online but haven’t had luck. The only product description is that it contains forest products. Is there a good resource for mulch? – Clackamas County

A: Manufacturers are typically unwilling to share proprietary information about their products, beyond “wood products.” There is no way to tell if the product is “safe” or not.  But this type of mulch is not recommended for vegetable gardens, anyway.  This answer explains why:

I’d suggest that you save the wood mulch for your ornamental landscape, and protect and warm the vegetable beds with those methods suggested in the above article. – Kris LaMar, OSU Extension Master Gardener

Need a list of native plants? Check here

Q: We are removing old plants and starting fresh with natives in our rural yard. Could someone help us with plant selection and soil assessment? – Clackamas County

A: The native plant recommendations depend on the conditions at your particular site. These three resources provide recommendations for many different types of sites.

List of resources for gardening with native plants: Native Plant Gardening | OSU Extension Service

Plant suggestions: Gardening with Oregon Native Plants West of the Cascades

This resource will help with plant selection for your particular site: OregonFlora Gardening with Natives Grow Natives Complete Collection

Soil testing can be done by OSU’s Soil Health Lab or by several private labs serving Oregon. The Extension office in your county can do soil pH testing as well.

A list of labs: Analytical Laboratories Serving Oregon

How do I test my soil? is a recent publication giving a general overview of soil testing. – Leo Sherry, OSU Extension Master Gardener

What’s the best grass seed to choke out weeds?

Q: I’m trying to tackle a big weed problem on my property. My goal is to plant grass seed this spring and I have someone coming to Hydroseed the hillside. He is asking what seed I want to plant. I’m hoping for the best grass seed to choke out the plethora of weeds, (Scotch broom, blackberries, poison hemlock) but also short in stature and really low maintenance since most of the planting will be done on a steep hillside. Do you have a suggestion for the correct seed to use? – Marion County

A: You have a couple of choices. Either plant a mix of fine fescues (red creeping, chewings and hard), or plant a mix of a lot of different grasses and see which ones survive.  

Highland bentgrass is another option you could add. Both of these will survive the summer without irrigation.   

Tall fescue is another option, but it won’t blend well with the other grasses and needs more nitrogen to do well. Perennial ryegrass won’t do well without summer irrigation.  

One thought, you might consider planting clover to “fix” nitrogen and therefore provides nitrogen to your grass. Clover will attract bees. So, if you don’t want bees, it’s not a good option. 

The challenges you have are that the grass will not be irrigated for three months in the summer, and you do not plan to mow it much. The mowing creates the density in the grass. Without frequent mowing, the grass will grow tall and lose density. Weeds will encroach and outcompete the grass in the summer. False dandelion tends to dominate unirrigated lawns. 

I don’t have much hope you will achieve your goals, but your best chance is to seed with a mix of fine fescue and highland bentgrass. You will have to buy the highland bentgrass online. And you are probably better off seeding this grass separately because the seed is like dust and only needs to be seeded at 1-2 pounds per 1,000 square feet. You can seed the fine fescue seed at 6 pounds per 1,000 square feet. 

– Brian McDonald, OSU Extension turf expert

What’s wrong with this azalea?

Q: My azaleas have developed a disease with the spotted leaves. The spots are small and fairly uniformly spaced. It seems like it might be a rust. The shrubs don’t seem to be getting worse, but they don’t flower well. What is it? Can I replace them with new azaleas without subjecting them to the disease? – Lane County

A: This is not a disease. This is the feeding damage of the azalea leaf bug (Stephanitis pyrioides) leaving small spots. On the underside of the leaf, you will see black spots. The red discoloration is a stress response to the feeding or to weather, but the leaf damage usually starts as yellow spots, followed by whitening of the leaves. This invasive pest is now quite common in our area and is most harmful on azaleas.

See this link from OSU Extension for more information. – Pat Patterson, OSU Extension horticulturist, retired

Is there any help for this oak with fungal rot?

Q: A branch about 20 feet high fell off my Oregon white oak tree and it is rotten and covered with mushrooms. I cut the rest of the branch off and the core is rotted. It looks like the rot may go partway into the tree. The rest of the tree appears healthy and there are no other visible mushrooms. Is there anything I can do to save it? If the answer is no, how long before it will rot enough that it becomes a hazard? – Clackamas County

A: There appear to be two types of fungal rot happening here. On the branch, the white rot most likely happened once the branch was already dead. The rot you see in the center is brown. The brown rot could be a fungus rotting out the oak’s heartwood.

Unfortunately, there are no treatments to prevent this or stop it. You can get a certified arborist out who can test the tree to see how rotted it is and, how much structural integrity and time the tree has left. – Alex Gorman, OSU Extension forester

Tree fertilization tips

Q: In the past I have used 16-16-16 fertilizer for my trees. When speaking with my brother-in-law, who is a retired arborist in Walla Walla, he indicated that he thought that to be excessive nitrogen and while he did not know our soils here, he suggested that I contact you folks for a recommendation.

My location is in the valley just north of Salem and the trees that I would be fertilizing include: katsura, maple, flowering cherry, paw paw, pear, medlar, weeping cedar, Scotch pine, weeping hemlock, yew and several other evergreens. – Marion County

A: Here’s some general info from the OSU Extension publication “Selecting, Caring, and Planting for a New Tree”:

“Fertilize established trees (1 year after planting) every two or three years in the fall after the leaves have dropped or in early spring before growth begins. Apply the fertilizer directly to the soil surface and water it in. If there is thick grass sod beneath the tree, use a pipe to punch holes 12 inches deep in the sod beneath the drip line of the tree and apply the fertilizer in the holes. This helps the fertilizer reach the tree’s root system. Avoid using “weed and feed” fertilizers around the root zone of your tree. Don’t apply nitrogen in late summer, because it can stimulate new growth that may not “harden off” or go into fall dormancy properly and will be more easily damaged by early fall frosts.”

– Brooke Edmunds, OSU Extension horticulturist

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